1. Field of the Invention
This invention relates to the field of current-driven emissive displays, and particularly to addressing and fabrication schemes suitable for use with such displays.
2. Description of the Related Art
A flat panel display is formed from an array of pixels arranged into rows and columns. Many technologies have been and continue to be explored in an effort to provide displays that offer high resolution, good panel brightness, and high reliability.
One approach that has shown promise involves the use of xe2x80x9corganic emissive devicesxe2x80x9d (OEDs). An OED pixel is formed by sandwiching an organic material between row and column address lines. An OED pixel is a current-driven device, having a brightness which varies directly and nearly instantaneously with its drive current. In contrast, liquid crystal device (LCD) pixels are voltage-driven devices, which respond comparatively slowly to the rms voltage applied across them.
Modern displays are typically required to provide a high information content, which requires the display to employ a large number of rowsxe2x80x94typically in excess of 1000xe2x80x94to provide the necessary resolution. There are two primary addressing schemes used with flat panel displays made from pixels arrayed into rows and columns: xe2x80x9cactive matrixxe2x80x9d and xe2x80x9cpassive matrixxe2x80x9d. Both schemes involve addressing every pixel in the array once per xe2x80x9cframe timexe2x80x9d. This requires providing excitation data (which determines whether the pixel is to be off or on, and in some cases its brightness level) to each pixel, which is typically accomplished by activating each row in turn from the top to the bottom of the array, and providing a data signal on each column line while a row is activated.
xe2x80x9cActive matrixxe2x80x9d addressing for an LCD display employs at least one transistor and a capacitor at each pixel site; a current conducted through the transistor charges the capacitor, which then maintains a drive signal to the pixel when the pixel""s row is not being addressed. Having one transistor and one capacitor at every pixel site results in a very high transistor count, a high fabrication cost, and possible reliability problems. This problem is exacerbated with an OED-type display, which, because the pixels are current driven, requires the use of a controllable current source at every pixel site, each of which requires at least two transistors and a capacitor. This further increases transistor count, and thereby worsens the associated cost and reliability problems.
xe2x80x9cPassive matrixxe2x80x9d addressing employs no active components at each pixel site; instead, the drive electronics reside at the end of each row and column. Thus, a pixel is only excited while it is being addressed. An OED display employing passive matrix addressing thus has a greatly reduced component count, but because a row""s pixels are not being excited while the other rows are being scanned, it is difficult to achieve a desired brightness level. There is an optimum voltage at which an OED pixel will operate efficiently. To increase the display""s brightness, an OED pixel can be driven with a voltage greater than the optimum, but doing so dramatically reduces the display""s energy efficiency and increases its heat output. If the display is battery-powered as on a laptop computer, for example, the inefficiency reduces the life of the battery, while the excess heat tends to shorten the lifetime of the OEDs.
An addressing scheme for use with current-driven emissive displays, and a fabrication scheme for use with all non-passive matrix displays, are presented which help to overcome the problems noted above. The addressing scheme provides performance superior to that provided by a passive matrix display, while requiring fewer components than are required by an active matrix display. A preferred fabrication scheme involves removing all active components from the pixel side of the display panel, serving to simplify display fabrication.
The addressing scheme requires that an array containing N rows and M columns of current-driven emissive display pixels be sub-divided into K segments, each of which consists of N/K rows and M columns. One controllable current driver is provided for each column of pixels within a segment, rather than for each pixel as with an active matrix. Each of the segments includes a respective gate address line, to which each of the segment""s current drivers is connected; each current driver is also connected to a corresponding column address line. Each of the current drivers within a segment is arranged to provide a current level to its column of pixels which corresponds to the voltage on the current driver""s corresponding column address line when the segment""s gate address line is selected. A controller is arranged to control the gate address lines as needed to cause the current drivers to control each of the pixels in the array.
The addressing of each pixel preferably proceeds as follows: a frame time is divided into N/K xe2x80x9csub-framexe2x80x9d times. During the first sub-frame time, the first segment""s current drivers are turned on and the first row of the first segment is addressed. The first rows of each of the remaining segments are addressed in turn during the remainder of the first sub-frame time. During the second sub-frame time, each of the second rows are addressed (one segment at a time), and the remaining rows are addressed in this manner during the remaining sub-frame times.
By segmenting and addressing the array in this way, the duty ratio required to drive the array is reduced by a factor of K when compared with a passive matrix display of the same size, and the number of transistors required to drive the array is reduced by a factor of N/K when compared with an active matrix display of the same size. These advantages are realized with arrays of any type of current-driven emissive pixel, such as OED pixels.
The present display can be implemented with the active components fabricated on the pixel side of the display panel. However, the fabrication of a current-driven emissive display, as well as other types of non-passive matrix displays, may be simplified by removing all active components from the pixel side of the display panel. Under this approach, the active components are placed either on the back side of the display panel or on a separate printed-circuit board (PCB). In the former case, the pixel array and drive electronics are formed on opposite sides of a multi-layer structure, with signals routed from the electronics to the pixel array directly through the structure. For the latter case, signals generated on the PCB are brought out to surface bonding pads on the board""s back side, and connection points for the pixels are brought out to surface bonding pads on the back side of the display panel. The PCB and display are arranged such that, when brought together, their respective bonding pads contact each other and thereby provide the necessary interconnections between PCB and display. Fabrication may be further simplified by combining the current drivers and other drive electronics into application-specific integrated circuits (ASICs).
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.